Membrane technology is a comparatively young technology for separating substance mixtures. Its basic principle consists in adding the substance mixture to be separated to a membrane, the membrane having a different permeability for the individual components present in the mixture. This leads to the components present in the substance mixture to be separated passing through the membrane (permeating) at different rates and accordingly becoming concentrated to differing degrees on both sides of the membrane. A separation criterion is therefore the permeability of the membrane with regard to the substance to be separated off. The driving force is mostly a pressure gradient between both sides of the membrane, the so-called transmembrane pressure Δp. In addition, other driving forces are also utilized.
Membrane technology utilizes here not only the mechanical sieving effect, which selects components according to differing particle sizes, but also solution and diffusion effects. A membrane thus acts in a considerably more complex manner than a simple mechanical filter and can therefore also separate liquids and/or gases from one another.
In the specific technical configuration, the mixture to be separated is conveyed as feed to the membrane. There, it is separated into a retentate on one side of the membrane and into a permeate on the other side of the membrane. Permeate and retentate are continuously drawn off from the membrane. On account of the separation effect, the components that become enriched in the permeate are those for which the membrane has a high permeability, whereas the substances that collect in the retentate are those for which the membrane is less permeable. Since many membrane processes use membranes which are in principle permeable for all components of the substance mixture—just with different rates of passage—such membrane processes do not separate digitally, but instead there are all of the components of the substance mixture both in the retentate as well as in the permeate, but in a different concentration (mass fraction).
Consequently, in membrane technology for the characterization of the permeability of a membrane, the retention R of the membrane as regards a specific component of the substance mixture is defined as follows:R:=1−wP/wR in which wP is the mass fraction of the component under consideration in the permeate and wR is the mass fraction of the component under consideration in the retentate of the membrane. The retention can therefore assume values from 0 to 1 and is therefore preferentially given in %. Looking at a simple two-component system, a retention of 0% thus for example means that the component under consideration permeates just as well as the solvent, meaning that the mass fractions of the component in the retentate are the same as in the permeate. On the other hand, a retention of 100 means that the component under consideration is completely retained.
Besides the retention as regards the component to be separated off, the so-called permeability of the membrane is also decisive for the characterization of its permeability:P:=m′/(A*Δp)in which m′ is the mass stream of the permeate, A is the area of the membrane and Δp is the applied transmembrane pressure. The permeability is usually stated in the unit kg/(h*m2*bar).
Permeability P and retention R are pregiven by the separation-active material of the membrane, and also by the composition of the substance mixture to be separated. These parameters are always relevant when designing a membrane separation process. On account of the substance dependency of these parameters, they are stipulated by the choice of membrane material. Consequently, the selection of separation-active membrane material for the particular separation task is decisive for the overall process design.
The principles of membrane technology reproduced hitherto and below can be consulted in Melin/Rautenbach: Membranverfahren. Grundlagen der Modul-und Anlagenauslegung. [Membrane Processes. Fundamentals of Module and System Design] Springer, Berlin Heidelberg 2004.
Depending on the parameter of the species to be separated off, the separation effects used and the driving force, a distinction is made between different classes of the membranes and/or of the membrane separation processes. The class's ultrafiltration, nanofiltration, gas permeation and reverse osmosis are customary. These concepts are not used uniformly and are not sharply delimited from one another. Whenever the discussion here is of a nanofiltration, what is intended is the separation off of molecules with a molar mass of more than 300 g/mol with the help of a membrane from an at least partially liquid substance mixture. Since such molecules have a diameter in the order of magnitude of 1 nm, the terms nanofiltration or nanofiltration membrane are used.
Depending on whether a predominantly aqueous substance mixture is separated or a predominantly organic substance mixture, the terms used are aqueous nanofiltration or organophilic nanofiltration. Since the resistance of the membrane materials and in particular their swelling behavior in the aqueous or organic medium prove to be very different, this distinction is relevant for the membrane technician.
One field of use in which organophilic nanofiltration can be used is the separation off of catalysts and/or their constituents or degradation products from reaction mixtures which originate from homogeneously catalyzed chemical reactions.
Catalysts serve to increase the rate of chemical reactions without thereby becoming consumed them in the process. In industrial chemistry, catalysts make the implementation of reactions economical or even possible in the first place. Consequently, catalysts are of great value in the chemical industry. Since the catalysts are not consumed, they can be separated off and reused after the reaction catalyzed therewith has concluded. The effort which has to be expended and/or is expended for catalyst separation depends on the value and the type of catalyst.
A distinction is made between two types of catalyst, namely homogeneous and heterogeneous. Homogeneous catalysts are in the same phase of the reaction mixture as the reaction participants, whereas heterogeneous catalysts are in a different phase. The heterogeneous catalysts are mostly solids which can be separated off easily from the liquid or gaseous reaction mixture. If desired, advantageously, this does not even require a sieve or filter since the solid catalyst simply remains in the reactor, while the liquid reaction mixture is drawn off. However, the separation of homogeneous catalysts, which are dissolved in the liquid reaction mixture, is complex. On account of the molecular state of the dissolved homogeneous catalyst, this is not possible with a filter. Consequently, a distillation, extraction or even a membrane separation is required. Since homogeneous catalysts reach particle sizes between 0.5 and 3 nm, a nanofiltration membrane is required for their separation.
One area of industrial chemistry in which organophilic nanofiltration can be used successfully in order to separate off valuable homogeneous catalyst from catalyzed liquid reaction mixtures is the hydroformylation of olefins (alkenes). During this synthesis, also called oxo reaction, olefins with n carbon atoms are reacted with a mixture of hydrogen and carbon monoxide—the so-called synthesis gas—to give aldehydes with n+1 carbon atoms.
The aldehydes are mostly converted to alcohols by hydrogenation, and, on account of their genesis, these are also termed oxo alcohols.
In principle, all olefins are amenable to hydroformylation, but in practice the substrates used in the hydroformylation are usually those olefins having two to 20 carbon atoms. Since alcohols obtainable by hydroformylation and hydrogenation have various possible uses—for instance as plasticizers for PVC, as detergents in washing compositions and as odorants—hydroformylation is practiced on an industrial scale.
The catalysts used in the hydroformylation are, inter alia, organometallic complex compounds which have a metal as central atom which is complexed with different ligands. Often, the ligands used are organophosphorous compounds, nonlimiting examples being organophosphines, organophosphites and organophosphoramidites. Such a catalyst system is dissolved in the liquid reaction mixture of olefin and synthesis gas dissolved therein and is therefore homogeneous. The separation takes place from the drawn-off reaction mixture comprising the formed aldehydes, by-products, unreacted starting materials and even dissolved homogeneous catalyst or constituents or degradation products thereof, thus pure metal, free ligands or degenerated metal and ligands, where degenerated metal can be understood as meaning, for example, multicore metal-carbonyl clusters, which are optionally still ligand-modified. Industrially, catalyst systems are used which have cobalt or rhodium as central atom, the latter often being complexed with organophosphorous ligands such as phosphine, phosphite or phosphoramidite compounds.
In particular, the separation of Rh-based catalyst complexes from homogeneously catalyzed hydroformylation mixtures has proven to be industrially demanding. One reason for this is that Rh is a very expensive noble metal, the loss of which should be avoided if possible. For this reason, the rhodium has to be separated substantially completely from the product stream and recovered. Since the Rh concentration in typical hydroformylation reactions is only 5 to 200 ppm and a typical “world scale” oxo plant achieves an annual output of 200 000 tonnes or more, separation apparatuses have to be used which, on the one hand, permit a large throughput and, on the other hand, safely separate off the Rh present only in small amounts. An additional complicating factor here is that the organophosphorous ligands belonging to the catalyst complex react very sensitively to state changes and rapidly degenerate. Since the decomposition products of the ligands are no longer able to stabilize either the organometallic complexes, or else bond particularly strongly to the metal centers, the sensitive equilibrium at the catalyst complex required for the successful catalytic reaction is disturbed. This leads, in macroscopic terms, to a deactivation of the catalyst. In the best case, a deactivated catalyst can be reactivated only in a costly and inconvenient manner. The catalyst therefore has to be separated off in a particularly gentle manner. A further important development aim is the capital costs of catalyst separation, so to speak of the nanofiltration plant.
Priske, M. et al. report on the possibilities of using membrane technology for processing hydroformylation mixtures: Reaction integrated separation of homogeneous catalysts in the hydroformylation of higher olefins by means of organophilic nanofiltration. Journal of Membrane Science, Volume 360, Issues 1-2, 15 Sep. 2010, pages 77-83; doi:10.1016/j.memsci.2010.05.002.
A more detailed overview of various solvent-based nanofiltration processes (Organic Solvent Nanofiltration, OSN) is given in the literature source: Chem. Soc. Rev., 2008, 37, 365-405.
A great advantage of the membrane separation processes compared to thermal separation processes is the lower energy input; however, in the case of membrane separation processes too, there is the problem of deactivation of the catalyst complex.
This problem has been solved by the process described in EP1931472B1 for the work-up of hydroformylation mixtures, in which a certain carbon monoxide partial vapor pressure is maintained both in the feed, in the permeate and also in the retentate of the membrane. It is thus possible for the first time to use membrane technology effectively in industrial hydroformylation.
A further example in the patent literature which deals with the membrane separation of homogeneous catalysts from hydroformylation mixtures is WO2014/183952A1. There, the retention of a membrane separation device is actively regulated, thus to steady the retentate volume stream and thus to ultimately counteract disturbances in the hydrodynamics of the reactor.
It is a specific disadvantage of membrane separation processes that this still comparatively young technology stands and falls with the availability of the membranes. In particular, the separation-active materials of the membrane which ultimately bring about the separation of the catalyst complexes and therefore critical to satisfying the separation task are still not available in large amounts or at low costs. The separation of large stream volumes, however, requires very large membrane areas and a correspondingly large amount of material and high capital costs. This leads to the technology, that has in the interim become ready industrially, cannot be used everywhere in an economical manner. This problem has hitherto not been solved in terms of process engineering. It relates not only to the hydroformylation, but also other homogeneously catalyzed reactions that are carried out on an industrial scale.
It is therefore an object to indicate an option as to how organophilic nanofiltration can be used for separating off homogeneous catalysts from reaction mixtures in an economical manner if the separation-active material of the membrane satisfying the separation task is not available in sufficiently large amounts or is available only at high cost.
This object is achieved through the consideration of a particular membrane performance indicator during the design and/or execution of the corresponding membrane separation process.